By David Shepardson

The United Auto Workers strike against the Detroit Three automakers is set to enter its third day Sunday with no immediate resolution on the horizon.

On Saturday, negotiators for the United Auto Workers and Ford Motor had "reasonably productive discussions" toward a new contract, the union said, while Chrysler-parent Stellantis said it had boosted its contract offer.

About 12,700 UAW workers remain on strike as part of a coordinated labor action targeting three U.S. assembly plants - one at each of the Detroit Three automakers after the prior four-year labor agreements expired at 11:59 p.m. ET Thursday.

Union negotiators and representatives of General Motors, Ford and Stellantis resumed talks on Saturday, a day after the UAW initiated the most ambitious U.S. industrial labor action in decades.

Stellantis said main bargaining talks are to resume Monday, while some subcommittee negotiations are set for Sunday at General Motors. UAW President Shawn Fain is scheduled to appear on two national news programs Sunday.

Stellantis said Saturday it hiked its offer, proposing raises of 20% over a four-and-a-half-year contract term, including an immediate 10% hike. That matches proposals from GM and Ford.

The proposals are about half the 40% wage hike the UAW is demanding through 2027, including an immediate 20% boost.

Mark Stewart, the North American chief operating officer for Stellantis, told reporters Saturday the UAW rejected a proposal to resume operations at an assembly plant in Belvidere, Illinois, noting its offer had been contingent on reaching agreement before the contract expiration.

In late February, Stellantis indefinitely idled operations at the Belvidere plant, citing rising costs of electric vehicle production.

The UAW criticized the company position on the Illinois plant saying now "they are now taking it back. That’s how they see these workers. A bargaining chip."

Stellantis said late Saturday is willing to negotiate about the plant's future. "The truth is UAW leadership ignored Belvidere in favor of a strike," the company said.

The strikes have halted production at three plants in Michigan, Ohio and Missouri that produce the Ford Bronco, Jeep Wrangler and Chevrolet Colorado, along with other popular models.

On Friday, Ford said it was indefinitely laying off 600 workers at a Michigan plant because of the impact of the strike at the facility, which makes the Bronco, and GM told some 2,000 workers at a Kansas car plant that their factory likely would be shut down Monday or Tuesday due to a lack of parts, stemming from the strike at a GM Missouri plant.

Besides higher wages, the UAW is demanding shorter work weeks, restoration of defined benefit pensions and stronger job security as automakers make the EV shift.

(Reporting by David Shepardson; Editing by Kim Coghill)